Gas detection complex and method for producing same, gas sensor comprising gas detection complex and method for manufacturing same

ABSTRACT

The inventive concept relates to a complex for detecting gas responsive to gas to be tested. The complex for the detecting the gas contains a nanostructure made of an oxide semiconductor, and a Terbium (Tb) additive supported on the nanostructure.

TECHNICAL FIELD

The inventive concept relates to a complex for detecting gas, a methodfor preparing the same, a gas sensor containing the complex fordetecting the gas, and a method for manufacturing the same.

BACKGROUND ART

An oxide semiconductor type gas sensor was first proposed by a professorSeiyama of Kyushu University in Japan and others in the 1960s. Sincethen, the oxide semiconductor type gas sensor has been widely used infields of driver's blood alcohol level measurement, explosive gasdetection, exhaust gas detection, indoor harmful gas detection, and thelike as the oxide semiconductor type gas sensor has various advantagesof being able to be integrated into a small size, being cheap, havinghigh sensitivity, being fast in response, and being able to detect gasconcentration as an electrical signal using a simple circuit. Inaddition, in recent years, as interests in human health andenvironmental pollution have increased, demands for indoor and outdoorenvironmental gas detection sensors, gas sensors for self-diagnosis ofdiseases, artificial olfactory sensors that may be mounted on mobiledevices, and the like are also rapidly increasing. However, the oxidesemiconductor type gas sensor has a fundamental problem of reactingeasily with external moisture, so that performance and reliabilitythereof are significantly degraded. Thus, it is still difficult tocommercialize the oxide semiconductor type gas sensor.

The oxide semiconductor type gas sensor detects gas through a change inresistance that occurs when reducing gas reacts with oxygen ionsadsorbed on an oxide surface. When the moisture is present in anatmosphere, the moisture consumes the oxygen ions on the oxide surfacefirst like gas to be tested, so that the sensor resistance changes andgas sensitivity decreases greatly. However, because the oxidesemiconductor type gas sensor operates in the atmosphere, exposure tothe moisture is not able to be avoided, and humidity varies greatlydepending on a weather, a season, and a region, so that it is almostimpossible to secure stable gas sensing characteristics without reducinghumidity dependence of the sensor. In particular, humidity of theatmosphere is generally about thousands to tens of thousands of ppm,which is very high compared to general concentration (several to tens ofppm) of the gas to be tested by the gas sensor. Thus, the changes in theresistance and in the sensitivity resulted from the humidity should beconsidered as the most important factor in securing reliability of thesensor. The remarkable performance and reliability degradation of theoxide semiconductor type gas sensor resulted from the humidity remainsan unsolved challenge for about 50 years since the gas sensor wasproposed, and acts as a major factor hindering the commercialization ofthe sensor. In other words, development of a highly reliable gas sensorthat exhibits constant gas sensitivity and sensor resistance regardlessof presence and concentration of the moisture is a first task that mustbe decided to increase the reliability of the sensor and to use thesensor in various applications.

Specifically, because the moisture in the atmosphere causes a reactionlike the gas to be detected on a surface of the oxide semiconductor, thesensor resistance and the gas sensitivity are greatly changed by thehumidity change, and the gas sensitivity decreases to one of several totens in a high humidity atmosphere, which are serious problems.Therefore, recently, humidity stability of a sensor material has emergedas the biggest issue in the field of the oxide semiconductor type gassensor. However, in equal to or more than 99% of existing studies on theoxide semiconductor type gas sensor, the gas sensing characteristics ina dry atmosphere are evaluated. In addition, there are few studies onthe gas sensor in the high humidity atmosphere. The reason why manystudies on such important issue have not been made is that the betterthe sensitive material, which has the high gas sensitivity, the higherthe reactivity with the moisture.

DETAILED DESCRIPTION OF THE INVENTION Technical Problem

The inventive concept is to provide a highly sensitive and highlyreliable complex for detecting gas and a gas sensor containing the samethat may accurately measure various gases regardless of presence andconcentration of moisture.

The problem to be solved by the inventive concept is not limitedthereto, and other problems that are not mentioned will be clearlyunderstood by those skilled in the art from a following description.

Technical Solution

The inventive concept provides a complex for detecting gas responsive tothe gas to be tested. According to an embodiment, the complex fordetecting the gas contains a nanostructure made of an oxidesemiconductor, and a Terbium (Tb) additive supported on thenanostructure.

The oxide semiconductor may be selected from a group consisting of tinoxide (SnO₂), zinc oxide (ZnO), and indium oxide (In₂O₃).

The nanostructure may have a hollow structure or an egg yolk structure.

The oxide semiconductor may be made of tin oxide (SnO₂), and the terbium(Tb) additive may be supported in an amount from 0.5 at % to 20 at %based on a total amount of tin (Sn) of the nanostructure.

The gas to be tested may be reducing gas selected from a groupconsisting of acetone, carbon monoxide, ammonia, toluene, xylene,benzene, and mixtures thereof.

In addition, the inventive concept provides a gas sensor for detectinggas. According to an embodiment, the gas sensor includes a substrate, asensing layer disposed on the substrate and containing a complex fordetecting gas responsive to the gas to be tested, and a terbium (Tb)layer disposed on the sensing layer, and the complex for detecting thegas contains a nanostructure made of an oxide semiconductor.

A thickness of the terbium (Tb) layer may be equal to or greater 50 nmand equal to or less than 250 nm.

The complex for detecting the gas may further contain a Terbium (Tb)additive supported on the nanostructure.

The oxide semiconductor may be selected from a group consisting of tinoxide (SnO₂), zinc oxide (ZnO), and indium oxide (In₂O₃).

The nanostructure may have a hollow structure or an egg yolk structure.

In addition, the inventive concept provides a complex for detecting gas.According to an embodiment, the method for preparing the complex fordetecting the gas includes a solution preparing step of preparingsolution containing at least one salt selected from a group consistingof tin (Sn) salt, zinc (Zn) salt, and indium (In) salt, terbium (Tb)salt, and an organic acid or sugar, an ultrasonic spray pyrolysis stepof performing an ultrasonic spray pyrolysis reaction by spraying thesolution through an ultrasonic spray pyrolysis apparatus, and anobtaining step of obtaining fine powders as a result of the ultrasonicspray pyrolysis reaction.

The tin (Sn) salt may be selected from a group consisting of SnC₂O₄,SnCl₄.xH₂O, and mixtures thereof, the zinc (Zn) salt may be selectedfrom a group consisting of Zn(NO₃)₂.xH₂O and mixtures thereof, theindium (In) salt may be selected from a group consisting ofIn(NO₃)₃.xH₂O and mixtures thereof, the terbium (Tb) salt may beselected from a group consisting of TbCl₃.6H₂O and mixtures thereof, theorganic acid may be selected from a group consisting of citric acid andmixtures thereof, and the sugar may be selected from a group consistingof sucrose and mixtures thereof.

The ultrasonic spray pyrolysis step may include spraying the solutionprepared from the solution preparing step into an electric furnaceheated at a temperature equal to or higher than 700° C. and equal to orlower than 1000° C. at a spray speed equal to or higher than 5 L/m andequal to or lower than 20 L/m.

Advantageous Effects of the Invention

The highly sensitive and highly reliable complex for detecting the gasand the gas sensor containing the same according to an embodiment of theinventive concept may accurately measure the various gases regardless ofthe presence and the concentration of the moisture.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic flowchart of a method for manufacturing egg yolkstructure gas sensors according to Present Example 1-1, Present Example1-2, Present Example 1-3, and Comparative Example 1 using ultrasonicspray pyrolysis.

FIG. 2 is a schematic flowchart of a method for manufacturing hollowstructure gas sensors according to Comparative Example 2, ComparativeExample 3, and Comparative Example 4 using ultrasonic spray pyrolysis.

FIG. 3 is a schematic flowchart of a method for manufacturing hollowstructure gas sensors including a Tb catalyst layer according to PresentExamples 2, 3, and 4 using ultrasonic spray pyrolysis.

FIG. 4 shows scanning electron microscope (SEM) photographs of egg yolkstructure fine powders of pure tin oxide (SnO₂) (Comparative Example 1,FIG. 4A), and SnO₂ doped with terbium (Tb) of 1 at % (Present Example1-3, FIG. 4B), 5 at % (Present Example 1-1, FIG. 4C), and 15 at %(Present Example 1-2, FIG. 4D) synthesized through ultrasonic spraypyrolysis.

FIG. 5 shows SEM photographs of hollow structure fine powders of tinoxide (SnO₂) (Comparative Example 2, FIG. 5A), zinc oxide (ZnO)(Comparative Example 3, FIG. 5B), and indium oxide (In₂O₃) (ComparativeExample 4, FIG. 5C).

FIG. 6 shows SEM images of a sensor sensitive film for ComparativeExample 2 (FIG. 6A), Comparative Example 3 (FIG. 6C), ComparativeExample 4 (FIG. 6E), Present Example 2 (FIG. 6B), Present Example 3(FIG. 6D), and Present Example 4 (FIG. 6F).

FIG. 7 is an analysis result of X-ray images of fine powders accordingto Comparative Example 1 (FIG. 7A), Present Example 1-1 (FIG. 7B),Present Example 1-2 (FIG. 7C), and Present Example 1-3 (FIG. 7D).

FIG. 8 is an analysis result of X-ray images of fine powders accordingto Comparative Example 2 (FIG. 8A), Present Example 2 (FIG. 8B),Comparative Example 3 (FIG. 8C), Present Example 3 (FIG. 8D),Comparative Example 4 (FIG. 8E), and Present Example 4 (FIG. 8F).

FIG. 9 shows graphs of sensor resistances (FIGS. 9A to 9D) and rates ofchange in the sensor resistance (FIGS. 9E to 9H) in a dry atmosphere andin an atmosphere of a relative humidity of 80% at a sensing targettemperature of 450° C. of Comparative Example 1, Present Example 1-1,Present Example 1-2, and Present Example 1-3.

FIG. 10 shows graphs of sensor sensitivities (FIGS. 10A to 10D) andrates of change in the sensor sensitivity (FIGS. 10E to 10H) in a dryatmosphere and in an atmosphere of a relative humidity of 80% at asensing target temperature of 450° C. of Comparative Example 1, PresentExample 1-1, Present Example 1-2, and Present Example 1-3.

FIG. 11 shows graphs of sensor resistances (FIGS. 11A to 11C) and ratesof change in the sensor resistance (FIGS. 11D to 11F) in a dryatmosphere and in an atmosphere of a relative humidity of 80% at asensing target temperature of 450° C. of Present Example 2, ComparativeExample 2, Present Example 3, Comparative Example 3, Present Example 4,and Comparative Example 4.

FIG. 12 shows graphs of sensor sensitivities (FIGS. 12A to 12C) andrates of change in the sensor sensitivity (FIGS. 12D to 12F) in a dryatmosphere and in an atmosphere of a relative humidity of 80% at asensing target temperature of 450° C. of Present Example 2, ComparativeExample 2, Present Example 3, Comparative Example 3, Present Example 4,and Comparative Example 4.

FIG. 13 shows graphs of sensor resistances (FIGS. 13A and 13B) and ratesof change in the sensor resistance (FIGS. 13C and 13D) in a dryatmosphere and in an atmosphere of a relative humidity of 80% at asensing target temperature of 450° C. of Present Example 5, ComparativeExample 5, Present Example 6, and Comparative Example 6.

FIG. 14 shows graphs showing gas sensitivities of fine powders based onacetone gas concentration according to Comparative Example 1 (FIG. 14A)and Present Example 1-1 (FIG. 14B).

FIG. 15 shows graphs of sensor sensitivities in a dry atmosphere and inan atmosphere of a relative humidity of 80% at 450° C. for 20 ppm ofvarious gases in Present Example 1-1 (FIG. 15A) and Comparative Example1 (FIG. 15B).

BEST MODE

Hereinafter, an embodiment of the inventive concept will be described inmore detail with reference to the accompanying drawings. Embodiments ofthe inventive concept may be modified in various forms, and a scope ofthe inventive concept should not be construed as being limited tofollowing embodiments. This embodiment is provided to more completelyexplain the inventive concept to those of ordinary skill in the art.Therefore, a shape of each of elements in the drawings is exaggerated toemphasize more clear description.

The inventive concept provides a complex for detecting gas responsive togas to be tested based on an oxide semiconductor nanostructure in whichterbium (Tb) is doped and/or added as a catalyst layer to solve theproblems of the prior art as described above. In this connection, theoxide semiconductor nanostructure plays a role of a main gas susceptorfor the gas to be tested, and the added terbium (Tb) selectively absorbsand removes moisture introduced from the outside.

It has been reported that the terbium (Tb) may play a role in furtherimproving an efficiency of CeO₂ used as a three-way catalyst forautomobiles by exhibiting excellent oxygen-accepting capacity throughexcellent valence conversion capability. Therefore, in the inventiveconcept, it was determined that a reverse reaction of following ReactionFormula 1, which is a moisture absorption reaction, may be induced whenthe terbium (Tb) is added to the gas susceptor. As a result of themeasurement, humidity dependence of a sensor actually decreased to aninsignificant level.

O⁻+H₂O→2OH+e ⁻  <Reaction Formula 1>

Therefore, the complex for detecting the gas according to an embodimentof the inventive concept contains the nanostructure made of the oxidesemiconductor and the terbium (Tb) additive supported on thenanostructure.

The nanostructure is selected from a group consisting of tin oxide(SnO₂), zinc oxide (ZnO), and indium oxide (In₂O₃). As will be describedlater, the nanostructure may be made of commercial nano fine powders,egg yolk structure fine powders, hollow structure fine powders, or thelike.

In addition, the terbium (Tb) additive may be used as a doped andcatalyst layer of the nanostructure for effective protection against themoisture and for minimization of an effect on a gas sensing reaction.

Furthermore, when being doped, the terbium (Tb) additive may besupported in an amount from 0.5 at % to 20 at % based on a total elementamount of tin (Sn) of the nanostructure. However, when the atomicpercent of the terbium (Tb) additive is less than 5 at %, the effect ofprotecting the gas susceptor from the moisture may be insignificant, andwhen the atomic percent of the terbium (Tb) additive exceeds 15 at %, aresistance of the sensor may be significantly increased. Therefore,preferably, when being doped, the terbium (Tb) additive may be supportedin the amount from 5 at % to 15 at % based on the total element amountof the tin (Sn) of the nanostructure.

In one example, the complex for detecting the gas according to theinventive concept may be used to detect various reducing gases selectedfrom a group consisting of acetone, carbon monoxide, ammonia, toluene,xylene, benzene, and mixtures thereof.

In addition, the inventive concept provides a method for preparing thecomplex for detecting the gas. The method according to the inventiveconcept includes:

a) a solution preparing step of preparing solution containing at leastone salt selected from a group consisting of tin (Sn) salt, zinc (Zn)salt, and indium (In) salt; terbium (Tb) salt; and an organic acid orsugar;b) an ultrasonic spray pyrolysis step of performing an ultrasonic spraypyrolysis reaction by spraying the solution through an ultrasonic spraypyrolysis apparatus; andc) an obtaining step of obtaining fine powders as a result of theultrasonic spray pyrolysis reaction.

Although not limited thereto, the tin (Sn) salt is selected from a groupconsisting of SnC₂O₄, SnCl₄.xH₂O, and mixtures thereof; the zinc (Zn)salt is selected from a group consisting of Zn(NO₃)₂.xH₂O and mixturesthereof; the indium (In) salt is selected from a group consisting ofIn(NO₃)₃.xH₂O and mixtures thereof; and the terbium (Tb) salt isselected from a group consisting of TbCl₃.6H₂O and mixtures thereof.

In this connection, when the terbium (Tb) is added to the nanostructurehaving the egg yolk structure using the tin (Sn) salt, the sugar isadded to the solution in step a). The sugar may be selected from a groupconsisting of sucrose and mixtures thereof.

In addition, when the terbium (Tb) is added to the nanostructure havingthe hollow structure using the tin (Sn) salt, the zinc (Zn) salt, andthe indium (In) salt, the organic acid and the sugar are added thereto.The organic acid may be selected from a group consisting of citric acidand mixtures thereof, and the sugar may be selected from a groupconsisting of the sucrose and the mixtures thereof.

In one example, the ultrasonic spray pyrolysis reaction may be performedunder a condition of spraying the solution prepared in step a) into anelectric furnace heated at a temperature between 700° C. and 1000° C. ata spray speed between 5 L/m and 20 L/m. In this step, fine droplets aregenerated by spraying the spray solution. A size of the fine droplet maybe controlled by a pressure inside the spray apparatus, concentration ofthe spray solution, viscosity of the spray solution, intensity of anultrasonic wave, and the like.

In addition, the inventive concept provides a gas sensor for detectinggas containing the complex for detecting the gas as a gas sensing layer.Such a gas sensor may be manufactured through

a) a complex solution preparing step of preparing solution containingthe complex for detecting the gas and a binder; andb) a sensing layer forming step of forming the sensing layer by coating,drying, and heat treating the solution on a substrate.

In this connection, the coating may be performed by a drop coatingprocess or a screen printing process.

The terbium (Tb) catalyst layer may be additionally deposited on thesensing layer of the gas sensor for detecting the gas. The terbium (Tb)catalyst layer may be deposited in a following method. For example, thecatalyst layer may be formed through electron beam evaporation,sputtering, or atomic layer deposition. Next, heating, that is, the heattreatment may be accompanied as needed to remove organic contaminantsand the like and to stabilize an incomplete phase. For example, aprocess of the heat treatment at a temperature between 100° C. and 600°C. may be performed.

Hereinafter, the inventive concept will be described in more detailthrough an embodiment, but a following embodiment is only intended toaid understanding of the inventive concept, and does not limit the scopeof the inventive concept.

Because the acetone is indoor and outdoor environmental pollutant gas,and at the same time, is biomarker gas that is detected from exhalationof a person suffering from diabetes, it is a very important toselectively detect the acetone regardless of presence or absence of themoisture. Therefore, in a following embodiment, the acetone was selectedas main gas to be measured, and effects of the external moisture on asensor resistance, a gas sensitivity, and the like of the sensor wereanalyzed.

In the inventive concept, gas sensors according to Comparative Examplesare manufactured using egg yolk structure fine powders (ComparativeExample 1) and hollow structure fine powders (Comparative Example 2) ofpure tin oxide (SnO₂), hollow structure fine powders of pure zinc oxide(ZnO) (Comparative Example 3), and hollow structure fine powders of pureindium oxide (In₂O₃) (Comparative Example 4). In addition, gas sensorsof tin oxide (SnO₂) egg yolk structures doped with the terbium (Tb) of 1at % (Present Example 1-3), 5 at % (Present Example 1-1), and 15 at %(Present Example 1-2) based on the total element amount of the tin (Sn),pure tin oxide (SnO₂) hollow structure on which the terbium (Tb)catalyst layer of 100 nm is deposited (Present Example 2), pure zincoxide (ZnO) hollow structure on which the terbium (Tb) catalyst layer of100 nm is deposited (Present Example 3), and pure indium oxide (In₂O₃)hollow structure on which the terbium (Tb) catalyst layer of 100 nm isdeposited (Present Example 4). For the gas sensors manufactured asdescribed above, the humidity dependences, the gas sensitivities, thesensor resistances, and the like of the sensors were compared with eachother. FIGS. 1 to 3 respectively illustrate a process chart of a methodfor manufacturing the egg yolk structure gas sensors according toPresent Example 1-1, Present Example 1-2, Present Example 1-3, andComparative Example 1 using the ultrasonic spray pyrolysis (FIG. 1), aprocess chart of a method for manufacturing the hollow structure gassensors according to Comparative Example 2, Comparative Example 3, andComparative Example 4 using the ultrasonic spray pyrolysis (FIG. 2), anda process chart of a method for manufacturing the gas sensors by coatingthe catalyst layers containing the terbium (Tb) on sensitive filmsaccording to Comparative Example 2, Comparative Example 3, andComparative Example 4 (FIG. 3).

Comparative Example 1, Present Example 1-1, Present Example 1-2, andPresent Example 1-3

First, 30 ml of hydrogen peroxide solution (H₂O₂, 30 wt % in H₂O) wasadded to 270 ml of distilled water to prepare final 300 ml of solution.Then, tin oxalate (SnC₂O₄) corresponding to 0.1 M was mixed with theprepared solution and then stirred for 24 hours. The sucrosecorresponding to 0.5 M was mixed with such solution and then stirred for5 minutes to prepare the spray solution. Tb chloride hexahydrate wasadded into the prepared spray solution such that an element ratio of tin(Sn)/terbium (Tb) is calculated to correspond to 100/0 (ComparativeExample 1), 99/1 (Present Example 1-3), 95/5 (Present Example 1-1), and85/15 (Present Example 1-2), stirred for 5 minutes, and thenultrasonically sprayed. The synthesized precursor was immediately heattreated while passing through the electric furnace (1000° C.) connectedto a spray outlet at the same time as being sprayed at a flow rate of 5L min⁻¹ (in O₂) to form tin oxide (SnO₂) egg yolk structures doped withthe terbium (Tb) of 0 at % (Comparative Example 1), 1 at % (PresentExample 1-3), 5 at % (Present Example 1-1), and 15 at % (Present Example1-2) based on the total element amount of the tin (Sn). The egg yolkstructure fine powders thus obtained were heat-treated at 600° C. for 2hours. The synthesized fine powders were mixed with tertiary distilledwater to form a mixture. The mixture was drop-coated on an aluminasubstrate on which a gold (Au) electrode was formed, and heat-treated at500° C. for 2 hours to manufacture the gas sensor. A change in theresistance of the manufactured sensor was measured while alternatelyinjecting, at 450° C., air in a dry atmosphere and air in an atmosphereof a relative humidity of 80%, or air+mixed gas in a dry atmosphere andair+mixed gas in the atmosphere of the relative humidity of 80% thereto.The gas was mixed in advance and then concentration thereof was rapidlychanged using a 4-way valve. A total flow rate was fixed at 200 sccm, sothat there was no temperature difference when the gas concentration waschanged.

Comparative Example 2 and Present Example 2

First, 3 mL of hydrochloric acid (HCl, between 35.0% and 37.0%) wasadded to 297 mL of the distilled water to form a mixture. 0.1 M of tinchloride and 0.025 M of citric acid monohydrate were added into themixture, stirred for 5 minutes, and then ultrasonically sprayed.Micro-sized droplets formed through the ultrasonic wave passed through a700° C. reactor at a flow rate of 20 L min⁻¹ (in air), and the tin oxide(SnO₂) hollow structure was formed (Comparative Example 2). The hollowstructure fine powders thus obtained are mixed with an organic binder toform a mixture. The mixture is screen-printed on the alumina substrateon which the gold (Au) electrode is formed, dried at 70° C. for 2 hours,and then heat-treated at 600° C. for 2 hours to produce a SnO₂ gassensitive film. Thereafter, the SnO₂ gas sensitive film was deposited tohave a thickness of 100 nm using a terbium source through an electronbeam evaporator, and then heat-treated at 550° C. for 2 hours tomanufacture a tin oxide (SnO₂) gas sensor on which the terbium (Tb) isapplied (Present Example 2). Gas sensitivity measurement of themanufactured gas sensor is the same as in Present Example 1-1, but onlythe measuring temperature was changed to 400° C.

Comparative Example 3 and Present Example 3

First, 0.2 M zinc nitrate hydrate was added to 300 mL of the distilledwater, stirred for 5 minutes, and then ultrasonically sprayed.Micro-sized droplets formed through the ultrasonic wave passed throughthe 700° C. reactor at the flow rate of 20 L min⁻¹ (in air), and thezinc oxide (ZnO) hollow structure was formed (Comparative Example 3).The hollow structure fine powders thus obtained were mixed with theorganic binder to form a mixture. The mixture is screen-printed on thealumina substrate on which the gold (Au) electrode is formed, dried at70° C. for 2 hours, and then heat-treated at 600° C. for 2 hours toproduce a zinc oxide (ZnO) gas sensitive film. Thereafter, the ZnO gassensitive film was deposited to have a thickness of 100 nm using theterbium source through the electron beam evaporator, and thenheat-treated at 550° C. for 2 hours to manufacture a ZnO gas sensor onwhich the Tb is applied (Present Example 3). The gas sensitivitymeasurement of the manufactured gas sensor is the same as in PresentExample 2.

Comparative Example 4 and Present Example 4

First, 0.05 M of indium nitrate hydrate and 0.15 M of the sucrose wereadded to 300 mL of the distilled water, stirred for 5 minutes, and thenultrasonically sprayed. Micro-sized droplets formed through theultrasonic wave passed through the 900° C. reactor at the flow rate of20 L min⁻¹ (in air), and the indium oxide (In₂O₃) hollow structure wasformed (Comparative Example 3). The hollow structure fine powders thusobtained were mixed with the organic binder to form a mixture. Themixture is screen-printed on the alumina substrate on which the gold(Au) electrode is formed, dried at 70° C. for 2 hours, and thenheat-treated at 600° C. for 2 hours to produce an indium oxide (In₂O₂)gas sensitive film. Thereafter, the In₂O₂ gas sensitive film wasdeposited to have a thickness of 100 nm using the terbium source throughthe electron beam evaporator, and then heat-treated at 550° C. for 2hours to manufacture an In₂O₃ gas sensor on which the terbium (Tb) isapplied (Present Example 4). The gas sensitivity measurement of themanufactured gas sensor is the same as in Present Example 2.

Comparative Example 5 and Present Example 5

First, tin oxide (SnO₂) commercial fine powders (Comparative Example 5)and Tb chloride hexahydrate were added into 20 mL of the distilled watersuch that the element ratio of the tin (Sn)/terbium (Tb) is calculatedto correspond to 95/5, and then, stirred at a temperature of 80° C. for2 hours. Such solution was dried in an electric oven at 70° C. for 24hours and then heat-treated in the electric furnace at 600° C. for 2hours (Present Example 5). Thereafter, the method for manufacturing thesensor and the gas sensitivity measurement were performed in the samemanner as in Present Example 1-1.

Comparative Example 6 and Present Example 6

First, indium oxide (In₂O₃) commercial fine powders (Comparative Example6) and the Tb chloride hexahydrate were added into 20 mL of thedistilled water such that an element ratio of indium (In)/terbium (Tb)is calculated to correspond to 97.5/2.5, and then, stirred at atemperature of 80° C. for 2 hours. Such solution was dried in theelectric oven at 70° C. for 24 hours and then heat-treated in theelectric furnace at 600° C. for 2 hours (Present Example 6). Thereafter,the method for manufacturing the sensor and the gas sensitivitymeasurement were performed in the same manner as in Present Example 1-1.

As a result of evaluation of gas sensing characteristics bymanufacturing the sensor with the fine powders synthesized in the methodas described above, all of Present Example 1-1, Present Example 1-2,Present Example 1-3, Present Example 2, Present Example 3, PresentExample 4, Present Example 5, Present Example 6, Comparative Example 1,Comparative Example 2, Comparative Example 3, Comparative Example 4,Comparative Example 5, and Comparative Example 6 showed n-typesemiconductor characteristics in which the resistance decreases at thesame time when the acetone is introduced after exhibiting a highresistance state in the air. Therefore, in a case of an n-type oxidesemiconductor, the gas sensitivity was defined as Ra/Rg (Ra: the sensorresistance in air, Rg: the sensor resistance in gas). Acetone sensingcharacteristics of each manufactured sensor were measured in the dryatmosphere and compared with acetone sensitivity and sensor resistancethereof measured in the atmosphere of the relative humidity of 80%. Adetailed measurement method is as follows.

After the sensor's resistance Ra became constant in air in the dryatmosphere, the atmosphere was suddenly changed to an atmosphere of thegas to be tested (from 10 ppm to 20 ppm of the acetone), and then thesensor was exposed to the gas to be tested. Thereafter, when theresistance in the gas became constant (Rg), the atmosphere was changedto the air in the dry atmosphere and then the gas sensitivity in the dryatmosphere was measured. Thereafter, the atmosphere was suddenly changedto the air with the atmosphere of the relative humidity of 80%. Thesensor was exposed to the gas to be tested with the relative humidity of80%, and gas sensing characteristics based on a first humidity wereevaluated. In addition, a value obtained by dividing the gas sensitivityof the atmosphere of the 80% relative humidity by the gas sensitivity ofthe dry atmosphere was defined as a rate of change in the gassensitivity. Further, a value obtained by dividing the sensor resistanceof the 80% relative humidity by the sensor resistance of the dryatmosphere was defined as a rate of change in the sensor resistance.Therefore, when the rate of changes in the gas sensitivity and thesensor resistance are 1, it may be determined that the sensor has littlehumidity dependence.

FIG. 4 shows scanning electron microscope (SEM) photographs of egg yolkstructure fine powders of pure tin oxide (SnO₂) (Comparative Example 1,FIG. 4A), and tin oxide (SnO₂) doped with terbium (Tb) of 1 at %(Present Example 1-3, FIG. 4B), 5 at % (Present Example 1-1, FIG. 4C),and 15 at % (Present Example 1-2, FIG. 4D) synthesized throughultrasonic spray pyrolysis. Referring to FIG. 4, it may be seen that allPresent Examples and Comparative Examples have the egg yolk structurewith the additional hollow structure defined therein, and sizes of thenanostructures are uniform.

FIG. 5 shows SEM photographs of hollow structure fine powders of tinoxide (SnO₂) (Comparative Example 2, FIG. 5A), zinc oxide (ZnO)(Comparative Example 3, FIG. 5B), and indium oxide (In₂O₃) (ComparativeExample 4, FIG. 5C). Referring to FIG. 5, it may be seen thatComparative Example 2, Comparative Example 3, and Comparative Example 4have the hollow structure with an empty interior.

FIG. 6 shows SEM images of a sensor sensitive film for ComparativeExample 2 (FIG. 6A), Comparative Example 3 (FIG. 6C), ComparativeExample 4 (FIG. 6E), Present Example 2 (FIG. 6B), Present Example 3(FIG. 6D), and Present Example 4 (FIG. 6F). Referring to FIG. 6, it maybe seen that the terbium catalyst layer is uniformly deposited on theupper sensitive film through the electron beam evaporator (FIGS. 6B, 6D,and 6F).

FIG. 7 is an analysis result of X-ray images of fine powders accordingto Comparative Example 1 (FIG. 7A), Present Example 1-1 (FIG. 7B),Present Example 1-2 (FIG. 7C), and Present Example 1-3 (FIG. 7D).Referring to FIG. 7, X-ray diffraction patterns of Comparative Example 1(FIG. 7A) and Present Examples 1-1 to 1-3 (FIGS. 7B to 7D) showed tinoxide (SnO₂) of a tetragonal structure. Although 1 at % and 5 at % ofthe terbium (Tb) were added in Present Example 1-3 (FIG. 7B) and PresentExample 1-1 (FIG. 7C), diffraction patterns associated with the terbium(Tb) were not identified. This is determined to be because an amount ofterbium (Tb) added was very small to fail to reach an analysis limit ofXRD (X-Ray Diffraction), or because the terbium (Tb) was evenly dopedthroughout the tin oxide (SnO₂) egg yolk structure. In addition, inPresent Example 1-2, terbium oxide (Tb₂O₃) and Tb₂ (Sn₂O₇) imagesappeared in addition to the tin oxide (SnO₂) of the tetragonalstructure.

FIG. 8 is an analysis result of X-ray images of fine powders accordingto Comparative Example 2 (FIG. 8A), Present Example 2 (FIG. 8B),Comparative Example 3 (FIG. 8C), Present Example 3 (FIG. 8D),Comparative Example 4 (FIG. 8E), and Present Example 4 (FIG. 8F).Referring to FIG. 8, in Present Example 2 (FIG. 8B) in which the Tbcatalyst layer was deposited on Comparative Example 2, diffractionpatterns of the tin oxide (SnO₂) and the terbium oxide (Tb₂O₃) of thetetragonal structures were shown.

Further, a diffraction pattern of Comparative Example 3 (FIG. 8C) showedzinc oxide (ZnO) of a hexagonal structure. Present Example 3 (FIG. 8D)in which the terbium (Tb) catalyst layer was deposited on ComparativeExample 3 showed diffraction patterns of the zinc oxide (ZnO) and theterbium oxide (Tb₂O₃) of the hexagonal structures.

In one example, a diffraction pattern of Comparative Example 4 (FIG. 8E)showed indium oxide (In₂O₃) of a cubic structure. Present Example 4(FIG. 8F) in which the terbium (Tb) catalyst layer was deposited onComparative Example 4 showed diffraction patterns of the indium oxide(In₂O₃) and the terbium oxide (Tb₂O₃) of the cubic structures.

FIG. 9 shows graphs of sensor resistances (FIGS. 9A to 9D) and rates ofchange in the sensor resistance (FIGS. 9E to 9H) in a dry atmosphere andin an atmosphere of a relative humidity of 80% at a sensing targettemperature of 450° C. of Comparative Example 1, Present Example 1-1,Present Example 1-2, and Present Example 1-3. Referring to FIG. 9, InComparative Example 1, when exposed to the atmosphere of the 80%relative humidity, the sensor resistance decreased by 36.4% compared tothe sensor resistance in the dry atmosphere (FIGS. 9A and 9E). This is ageneral phenomenon that occurs when an n-type oxide semiconductor typegas sensor such as the tin oxide (SnO₂) is exposed to the moisture, andis a major factor that degrades a performance of the sensor and causesmalfunction. On the other hand, in Present Example 1-1, the sensorresistance decreased by 24.3% compared to the sensor resistance in thedry atmosphere even when suddenly exposed to the atmosphere of the 80%relative humidity (FIGS. 9B and 9F). In Present Example 1-2, similarly,the sensor resistance was 102.5% of a sensor resistance value in the dryatmosphere, which was almost similar, but measurement may becomesomewhat difficult as the sensor resistance increases (FIGS. 9C and 9G).On the other hand, in Present Example 1-3 doped with the 1 at % terbium(Tb), when exposed to the atmosphere of the 80% relative humidity, theresistance of the sensor was 65.4% of that in the dry atmosphere (FIGS.9D and 9H).

FIG. 10 shows graphs of sensor sensitivities (FIGS. 10A to 10D) andrates of change in the sensor sensitivity (FIGS. 10E to 10H) for 20 ppmof the acetone in a dry atmosphere and in an atmosphere of a relativehumidity of 80% at a sensing target temperature of 450° C. ofComparative Example 1, Present Example 1-1, Present Example 1-2, andPresent Example 1-3. In Comparative Example 1, the sensor sensitivityfor 20 ppm of the acetone in the atmosphere of the 80% relative humiditywas 30.7, which was reduced to be 63.6% of the sensor sensitivity of63.9 in the dry atmosphere (FIGS. 10A and 10E). In contrast, in PresentExample 1-1, the sensor sensitivity in the atmosphere of the 80%relative humidity was maintained at 12.0, which was 80.6% of the sensorsensitivity of 16 in the dry atmosphere (FIGS. 10B and 10F). In PresentExample 1-2, the rate of change in the sensor sensitivity was 82%, whichwas not significantly affected by the humid atmosphere (FIGS. 100 and10G). In Present Example 1-3, the sensor sensitivity in the atmosphereof the 80% relative humidity was 9.9, which was increased to be 107.6%of 9.2 in the dry atmosphere.

Considering the rate of change in the sensor resistance, the rate ofchange in the sensor sensitivity, the sensitivity of the sensor, and theresistance of the sensor in the dry atmosphere and in the atmosphere ofthe 80% relative humidity with reference to FIGS. 9 and 10, it may beseen that Present Example 1-1 shows constant gas sensing characteristicswith high reliability regardless of the presence or the absence of theexternal moisture, and Present Example 1-2 and Present Example 1-3significantly reduce humidity dependence of the sensing characteristicscompared to Comparative Example 1.

FIG. 11 shows graphs of sensor resistances (FIGS. 11A to 11C) and ratesof change in the sensor resistance (FIGS. 11D to 11F) in a dryatmosphere and in an atmosphere of a relative humidity of 80% at asensing target temperature of 450° C. of Present Example 2, ComparativeExample 2, Present Example 3, Comparative Example 3, Present Example 4,and Comparative Example 4.

In Comparative Example 2, when exposed to the atmosphere of the 80%relative humidity, the sensor resistance decreased significantlycompared to that in the dry atmosphere, so that the rate of change inthe sensor resistance was 82.6%, and in Present Example 2, theresistance was not significantly reduced even in the humid atmosphere,thereby exhibiting the rate of change in the sensor resistance of 16.2%(FIGS. 11A and 11D).

In Comparative Example 3, when exposed to the atmosphere of the 80%relative humidity, the sensor resistance decreased significantlycompared to that in the dry atmosphere similar to Comparative Example 2,so that the rate of change in the sensor resistance was 66.7%, and inPresent Example 3, the resistance was not significantly reduced even inthe humid atmosphere, thereby exhibiting the rate of change in thesensor resistance of 19.0% (FIGS. 11B and 11E).

Also, in Comparative Example 4, when exposed to the atmosphere of the80% relative humidity, the sensor resistance decreased by 59.0% comparedto that in the dry atmosphere, and in Present Example 4, the sensorresistance decreased by 19.0% in the humid atmosphere compared to thatin the dry atmosphere (FIGS. 11C and 11F).

FIG. 12 shows graphs of sensor sensitivities (FIGS. 12A to 12C) andrates of change in the sensor sensitivity (FIGS. 12D to 12F) of 10 ppmof acetone in a dry atmosphere and in an atmosphere of a relativehumidity of 80% at a sensing target temperature of 450° C. of PresentExample 2, Comparative Example 2, Present Example 3, Comparative Example3, Present Example 4, and Comparative Example 4.

In Comparative Example 2, the sensor sensitivity for 10 ppm of theacetone in the atmosphere of the 80% relative humidity was 8.8, whichwas reduced to be 80.6% compared to the sensor sensitivity of 45.7 inthe dry atmosphere. On the other hand, in Present Example 2, the sensorsensitivity in the atmosphere of the 80% relative humidity was 34.5,which was 55.5% increased from the sensor sensitivity of 22.3 in the dryatmosphere (FIGS. 12A and 12D).

Similarly, in Comparative Example 3 and Comparative Example 4, thesensor sensitivities for 10 ppm of the acetone in the atmosphere of the80% relative humidity were 18.1 (Comparative Example 3) and 20.8(Comparative Example 4), respectively, which were significantly reducedfrom the sensor sensitivities of 116.2 (Comparative Example 3) and 44.4(Comparative Example 4) in the dry atmosphere, respectively (FIGS. 12B,12C, 12E, and 12F). In contrast, in Present Example 3 and PresentExample 4, the sensor sensitivities for 10 ppm of the acetone in theatmosphere of the 80% relative humidity were 18.1 (Comparative Example3) and 20.8 (Comparative Example 4), respectively, which were increasedor constant compared to the sensor sensitivities of 6.2 (ComparativeExample 3) and 20.8 (Comparative Example 4) in the dry atmosphere,respectively (FIGS. 12B, 12C, 12E, and 12F).

Referring to FIGS. 11 and 12, when the terbium (Tb) catalyst layer iscoated on the sensor sensitive film, the rates of change in the sensorresistance and in the sensor sensitivity in the dry atmosphere and inthe atmosphere of the 80% relative humidity may be adjustable. Thismeans that a sensor that exhibits the constant gas sensingcharacteristics with the high reliability regardless of the presence orthe absence of the external moisture may be designed.

FIG. 13 shows graphs of sensor resistances (FIGS. 13A and 13B) and ratesof change in the sensor resistance (FIGS. 13C and 13D) in a dryatmosphere and in an atmosphere of a relative humidity of 80% at asensing target temperature of 450° C. of Present Example 5, ComparativeExample 5, Present Example 6, and Comparative Example 6.

In Comparative Example 5, when exposed to the atmosphere of the 80%relative humidity, the sensor resistance decreased significantlycompared to that in the dry atmosphere, so that the rate of change inthe sensor resistance was 37.8%, and in Present Example 5, theresistance was not significantly reduced even in the humid atmosphere,thereby exhibiting the rate of change in the sensor resistance of 14.4%(FIGS. 13A and 13C).

In Comparative Example 6, when exposed to the atmosphere of the 80%relative humidity, the sensor resistance decreased significantlycompared to that in the dry atmosphere similar to Comparative Example 2,so that the rate of change in the sensor resistance was 36.6%, and inPresent Example 6, the resistance was not significantly reduced even inthe humid atmosphere, thereby exhibiting the rate of change in thesensor resistance of 16.0% (FIGS. 13B and 13D).

FIG. 14 shows graphs showing gas sensitivities of fine powders based onacetone gas concentration according to Comparative Example 1 (FIG. 14A)and Present Example 1-1 (FIG. 14B). Compared to Present Example 1-1(14B), Comparative Example 1 (14A) showed different gas sensitivitiesbased on the concentration of the acetone in the dry atmosphere and inthe atmosphere of the 80% relative humidity. The gas sensitivity and thesensor resistance at this time showed characteristics that are hardlyaffected by the presence or the absence of the moisture. The acetone isa gas that pollutes an environment, and at the same time, is also abiomarker gas of which 300 to 900 ppb is detected in exhalation ofnormal people and 1800 ppb or more is detected in exhalation of diabeticpatients. Therefore, when the acetone of roughly 1 ppm is able to beselectively detected irrespective of the presence or the concentrationof the moisture, the acetone may be used for a diabetes self-diagnosissensor. Because an acetone measurement limit of Present Example 1-1 isat least 50 ppb, it is expected that the gas sensor according to theinventive concept may be sufficiently utilized for a disease diagnosissensor by exhalation for diabetes self-diagnosis and the like.

FIG. 15 shows graphs of gas sensitivities at 450° C. for 20 ppm ofvarious gases (acetone, ammonia, carbon monoxide, toluene, xylene, andbenzene) in Present Example 1-1 (FIG. 15A) and Comparative Example 1(FIG. 15B). It may be seen that humidity dependences for various gasesin the dry atmosphere and in the atmosphere of the 80% relative humidityare significantly reduced in Present Example 1-1 compared to ComparativeExample 1.

The foregoing detailed description illustrates the inventive concept.The foregoing is also illustrative of the preferred embodiments of theinventive concept, and the inventive concept may be used in variousother combinations, modifications and environments. That is, the scopeand the description of the inventive concept disclosed in thisspecification may be changed or modified within the scope of equivalentsand/or the skill or knowledge of the inventive concept. The embodimentsdescribed above are intended to explain certain best modes forimplementing the technical idea of the inventive concept. Variousmodifications required for the specific application and usage of theinventive concept are possible. Therefore, the detailed description ofthe inventive concept is not intended to limit the inventive concept tothe disclosed embodiments. It is also to be understood that the appendedclaims are intended to cover further embodiments.

1. A complex for detecting gas, the complex containing: a nanostructuremade of an oxide semiconductor; and a Terbium (Tb) additive supported onthe nanostructure.
 2. The complex of claim 1, wherein the oxidesemiconductor is selected from a group consisting of tin oxide (SnO₂),zinc oxide (ZnO), and indium oxide (In₂O₃).
 3. The complex of claim 1,wherein the nanostructure has a hollow structure or an egg yolkstructure.
 4. The complex of claim 1, wherein the oxide semiconductor ismade of tin oxide (SnO₂), wherein the terbium (Tb) additive is supportedin an amount from 0.5 at % to 20 at % based on a total amount of tin(Sn) of the nanostructure.
 5. The complex of claim 1, wherein the gas tobe tested is reducing gas selected from a group consisting of acetone,carbon monoxide, ammonia, toluene, xylene, benzene, and mixturesthereof.
 6. A gas sensor for detecting gas, the gas sensor comprising: asubstrate; a sensing layer disposed on the substrate and containing acomplex for detecting gas responsive to the gas to be tested; and aterbium (Tb) layer disposed on the sensing layer, wherein the complexfor detecting the gas contains a nanostructure made of an oxidesemiconductor.
 7. The gas sensor of claim 6, wherein a thickness of theterbium (Tb) layer is equal to or greater 50 nm and equal to or lessthan 250 nm.
 8. The gas sensor of claim 6, wherein the complex fordetecting the gas further contains a Terbium (Tb) additive supported onthe nanostructure.
 9. The gas sensor of claim 6, wherein the oxidesemiconductor is selected from a group consisting of tin oxide (SnO₂),zinc oxide (ZnO), and indium oxide (In₂O₃).
 10. The gas sensor of claim6, wherein the nanostructure has a hollow structure or an egg yolkstructure.
 11. A method for preparing a complex for detecting gas, themethod comprising: a solution preparing step of preparing solutioncontaining at least one salt selected from a group consisting of tin(Sn) salt, zinc (Zn) salt, and indium (In) salt, terbium (Tb) salt, andan organic acid or sugar; an ultrasonic spray pyrolysis step ofperforming an ultrasonic spray pyrolysis reaction by spraying thesolution through an ultrasonic spray pyrolysis apparatus; and anobtaining step of obtaining fine powders as a result of the ultrasonicspray pyrolysis reaction.
 12. The method of claim 11, wherein the tin(Sn) salt is selected from a group consisting of SnC₂O₄, SnCl₄.xH₂O, andmixtures thereof, wherein the zinc (Zn) salt is selected from a groupconsisting of Zn(NO₃)₂.xH₂O and mixtures thereof, wherein the indium(In) salt is selected from a group consisting of In(NO₃)₃.xH₂O andmixtures thereof, wherein the terbium (Tb) salt is selected from a groupconsisting of TbCl₃.6H₂O and mixtures thereof, wherein the organic acidis selected from a group consisting of citric acid and mixtures thereof,and wherein the sugar is selected from a group consisting of sucrose andmixtures thereof.
 13. The method of claim 11, wherein the ultrasonicspray pyrolysis step includes: spraying the solution prepared from thesolution preparing step into an electric furnace heated at a temperatureequal to or higher than 700° C. and equal to or lower than 1000° C. at aspray speed equal to or higher than 5 L/m and equal to or lower than 20L/m.